1. Technical Field
The present invention relates to a sheet folding apparatus that folds a sheet with an image formed thereon, and more particularly, to a sheet folding apparatus for enabling a sheet to be folded in an accurate fold position with a simplified structure and an image formation system provided with the apparatus.
2. Description of Related Arts
Generally, this type of sheet folding apparatus has been known as an apparatus for folding a sheet with an image formed thereon by an image formation apparatus such as a printing press, printer apparatus and copier in a predetermined fold position to perform finish processing, and for example, there are an apparatus which is coupled to a sheet discharge outlet of an image formation apparatus, folds a sheet with an image formed for filing, and carries the sheet out to a subsequent binding processing apparatus and the like. The sheet folding apparatus for thus folding a sheet in half or one-third to carry out is configured as a post-processing apparatus of the image formation apparatus, or as a unit incorporated into the image formation apparatus or binding processing apparatus.
As a folding form in such a sheet folding apparatus, for example, for filing, various forms such as ½ folding, ⅓ Z-folding and ⅓ letter-folding are known corresponding to the intended use, and in forms such as ⅓ Z-folding and ⅓ letter-folding with the need of folding in three among the forms, the folding processing is performed twice.
Then, in the case of performing the folding processing twice, for first folding, since a sheet is transported while being held and regulated by the transport means, it is easy to accurately detect a fold position of first folding by measuring timing after detecting the front end of the sheet. However, in second folding, since the sheet, which is first folded and fed in a relatively free state, is nipped and second folding is formed, there is the problem that fluctuations occur in the fold position in nipping and that an accurate fold is not formed.
Therefore, there is a configuration in which the front end of the sheet transported to form second folding is struck by a stopper to add regulation to the sheet, and the sheet is thereby nipped in a certain position to perform the fold processing. However, in such a sheet folding apparatus using the stopper, since the front end of the sheet is struck by the stopper and regulated, the distance between the stopper and a folding roller pair is relatively long, and there is the problem that the sheet tends to be slack in between the stopper and the folding roller pair.
Therefore, such a configuration is known that a sensor is provided on the downstream side of a folding roller pair in the sheet transport direction, the sheet is halted by break means when the sensor detects that the sheet reaches downstream by a predetermined distance, and that a loop of the sheet occurring by continuing transport of the sheet is nipped with the folding roller pair to form the fold (for example, see Japanese Patent Application Publication No. 2000-44115).
Another configuration is known in which a roller pair capable of rotating forward and backward is provided on the downstream side of a folding roller pair, the roller pair rotates forward and holds a sheet when the front end of the sheet comes into contact with the halted roller pair, and then rotates backward this time to form a loop in the sheet, and the folding roller pair nips the loop to form the fold (for example, see Japanese Patent Application Publication No. 2006-76776).